1. Field of the Invention
The invention pertains to multilayer printed circuit boards and to corresponding fabrication methods.
2. Description of the Related Art
In prior multilayer printed circuit boards, it is common to dispose power supply and ground layers inside and signal layers outside, and to interconnect those layers by through holes. Inasmuch as the through holes are formed by mechanical drilling and hence tend to be of relatively large size, it is difficult to increase the wiring density of the circuit board. Also, inasmuch as it is necessary to design the signal layers on both sides while always taking through hole connections into consideration, freedom in wiring design is relatively restricted. Moreover, because the through holes pass through the power supply and ground layers, it is necessary to provide clearance holes at those through hole locations in the power supply and ground layers where no power supply or ground connections are required. Accordingly, the higher the through hole density becomes, the smaller the clearance holes have to be formed with still higher accuracy, which in turn entails more accurate alignment of the signal layers with the power supply and ground layers in laminating those layers. Recently, in addition to PIH (Pin In Hole) components which are mounted by inserting their pins into through holes, SMT (Surface Mount Technology) components which are mounted without pins have come to be used. Therefore, there is a need for multilayer printed circuit boards which can be applied to both SMT and PIH components, and which can provide higher wiring density and higher degree of freedom in design of wiring.